If this kind of army is going to be marched into Jerusalem, I doubt that this would be an actual military campaign. It will probably be more of a tour d'force. When the First Crusaders passed through Constantinople they swore allegiance to the Byzantine Emperor, an oath they quickly walked away from. If John II is going to march an army into Jerusalem, then I think that he might resurrect those oaths, and force the Latin East to swear fealty to him. This might be a bit much, and would probably cause more problems than it would solve. Additionally, John II knows that he cannot maintain control over the Latin East if the Latin lords oppose him.
John II's main objective was to force Antioch into the Empire. Antioch was a natural extension of John's conquest of the Anatolian coast, and would provide a base from which to wage war against the Iconium Sultanate. Unfortunately for the Byzantines Antioch was not so keen on the idea of becoming a part of the Byzantine Empire. There was a plan in 1138 for a joint Byzantine-Antioch campaign to conquer Aleppo. The idea was that once Aleppo had fallen, it would be awarded to Prince Raymond, who would give up Antioch to the Byzantines. The plan failed due to Raymond's opposition to giving up Antioch. If John II comes into the Latin East with a whole army, he might be able to get the Latin East to attack Aleppo, and then force the exchange with Raymond. In this situation John II would be betting that the Latin East would be willing to sacrifice Antioch if it meant keeping the Byzantine Army out of Jerusalem.